The transmission of optical signals between optical fibers and integrated optical waveguides poses several technological challenges, and various approaches have been suggested to improve light coupling efficiency. One type of approach uses diffraction gratings to couple light vertically, or nearly vertically, between a single-mode optical fiber and an integrated waveguide. In a typical configuration, the optical fiber is disposed vertically, or nearly vertically, over the waveguide and a diffraction grating is disposed on or near the surface of the waveguide for directing light from and/or to the optical fiber. Such a configuration usually results in a large overall package footprint, which can prevent or limit miniaturization and associated cost reduction.
An approach to alleviate these limitations is to provide the optical fiber with an angled end to define a reflecting surface configured to laterally couple light between the core of the optical fiber and the diffracting grating coupler. One drawback of this approach is that the light reflected by the reflecting surface is coupled out of the core as a diverging light beam whose cross-sectional area increases as it propagates laterally outwardly through the cladding and toward the diffraction grating. A similar situation arises for light coupled from the diffracting grating toward the fiber. In a standard single-mode fiber with a cladding diameter of 125 micrometers (μm), the reflecting surface and the diffraction grating are separated from each other by at least 62.5 μm. Such a distance can be sufficiently large to cause mode size mismatch at the diffraction grating, resulting in optical power losses that degrade the coupling efficiency. Other approaches have attempted to overcome or at least mitigate this beam-divergence-induced mismatch issue, for example by providing a non-flat reflecting surface or by thinning or tapering the fiber cladding to bring the fiber core closer to the diffraction grating. In such approaches, each fiber is manufactured individually using high-precision machining or polishing processes. Consequently, various challenges remain in the field of optical fiber coupling techniques.